INDEX
1.
INTRODUCTION.................................................................................................3
2.
OBJECTIVE........................................................................................................3
3.
CODES AND STANDARDS...................................................................................3
4.
ELECTRICAL SYSTEM DETAILS...........................................................................3
5.
CABLE INSTALLATION........................................................................................4
6.
SIZING CALCULATIONS......................................................................................4
7.
LV CABLE SIZING...............................................................................................4
8.
REFERENCE DOCUMENTS.................................................................................9
9.
TYPICAL CALCULATIONS..................................................................................10
10.
NOTES............................................................................................................12
1.
INTRODUCTION
2.
OBJECTIVE
The Objective of this document is to define the Minimum cable sizing requirement for LV AC
power , control cables and DC cables.
3.
CODES AND STANDARDS
IEC
International Electro Technical Commission Standards
BS
British Standards
IEC-60287
Calculation of current ratings
IEC-60364-4-43
Low Voltage Electrical Installations
IEC-60364-5-52
Selection and Erection of Electrical Equipment-Wiring Systems
IEC-60502-2
Power Cables with Extruded insulation and their accessories
for rated voltages from 1KV upto 30KV
4.
IEC-60228
Conductors of insulated cables
IEC-60364-5-54
Electrical installations of buildings
BS-7671
Requirements for Electrical installations
ELECTRICAL SYSTEM DETAILS
LV System
Frequency
System Neutral Earthing at
supply end
Design Fault level
5.
415/240 V
50 Hz
Solidly Earthed
50kA for 1sec
CABLE INSTALLATION
The methods of cable installation shall be as per " General Electrical Specification, Electrical
Requirements for Packaged Equipment, Low Voltage Power & Control Cables Specification.
6.
SIZING CALCULATIONS
Cables shall be sized in accordance with the following criteria and the relevant standards listed in
this document.
7.
Continuous current carrying capacity.
Short Circuit withstand criteria.
Steady state voltage drop.
Motor starting voltage drop.
Grouping of cables.
Method of cable laying / installation.
LV CABLE SIZING
7.1
Continuous Current carrying capacity
The continuous current carrying capacity of a cable corresponds to the maximum current that it
can carry under specified conditions without the conductors exceeding the permissible limit of
steady state temperature for the type of insulation concerned. Derating factor or correction factor
shall be applied where the installation conditions differ from those for which values of current
carrying capacity are defined.
The current carrying capacities for XLPE multi core cables are considered as per Oman Cables
Industry LV Power & Control Catalogue which is as per IEC 60364-5-52 standard along with the
following parameters.
7.2
Design temperature for outdoor equipment
55 C
Max. Conductor Operating temperature:
90 C
Correction Factors
The various correction factors under different conditions like variation of Ground temperature, Soil
thermal resistivity, grouping of cables and variation in ambient temperature are considered as per
Oman Cables Industry-LV Power & Control Catalogue (Appendix-2) which is as per IEC 60364-552 standard.
As per the Oman Cables Industry LV Power & Control Catalogue the derating factors for LV
cables laid in ground are considered.
a
b
Rating factor for ambient air temperature of 55 C
Rating factor for grouping of cables (6 cables with
0.15m spacing)
Full Load current (FLA) Ib
Kt
= 0.94
Kg
=0.57
Load (KW) x1000
3 System Voltage x Power Factor x efficiency
(For 3phase AC loads)
Full Load current (FLA) Ib
Load (KW) x1000
System Voltage x Power Factor x efficiency
(For 1phase AC loads)
Full Load current (FLA) Ib
Load (KW) x1000
System Voltage x efficiency
(For DC loads)
Cable Site Rating = (Cable Current Carrying Capacity) x (Group Derating Factor)
x (No. of Conductor per phase)
7.3
Resistance and Reactance of cables
Appendix-3 provides resistance and reactance values of various sizes for Copper, XLPE, SWA
cables suitably taken from Approved Cable vendors list. (Referred Oman Cables Industry LV
Power & Control Catalogue).
7.4
Short Circuit withstand criteria
Minimum cross section of cable, based on short circuit
The cable short circuit capacity is evaluated as
ISC2 = K2S2 loge 1 +
T
0 +
Where
ISC
T
K
S
1
0
= short circuit current (rms) in Amp
= duration of short circuit in seconds
= constant for material of conductor
= area of conductor in sqmm
= final temperature in C.
= initial temperature in C.
= reciprocal of temperature coefficient of resistance of conductor in C at 0C.
The constants K and are considered as per IEC-60364-4-43 for Copper/Aluminium cables and
1 and 0 for Copper / XLPE cable are:
1 = 250 C (for XLPE cable)
0 = 90 C (for XLPE cable) respectively
on substituting the above values in the given formula, the short circuit currents for
Copper, XLPE cable is
ISC = 143 * S
T
and for
Aluminium XLPE cable is
ISC = 94 * S
T
and minimum cross section
for Copper XLPE cable shall be derived as S = ISC * T
143
and for
Aluminium XLPE cable shall be derived as S = ISC * T
94
Protective devices either fuses or circuit breakers shall be provided to break any short-circuit
current flowing in the conductors before such a current could cause a danger due to thermal and
mechanical effects produced in the conductors and connections.
The cables shall have a minimum section, so that it will be able to support, without being
damaged all thermal stress derived from short-circuit current for all duration time, to be identified
as intervention time of the protection device.
According to IEC 60364-4-43 the cable sizing shall satisfy the following condition, valid for short
circuit.
K2S2 ISC2 t
Where,
K2 S2
= cable passing through energy
I sc2 t
= protection device let-through energy (A2s)
= coefficient depending on the electrical and thermal characteristics of the cables,
initial and final temperature as per IEC 60364-4-43:
= 94 for Aluminium and 143 for Copper XLPE insulated cables
= cross-sectional area in sq mm.
I sc
= effective short-circuit current in Amp expressed as r.m.s. value.
= duration in seconds of the short circuit depending of protective device that shall be
provided to break the short circuit current flowing in the circuit (tripping time).
7.5
Voltage Drop Calculation
7.5.1 Allowable Voltage Drop
Voltage drop during motor starting conditions shall be limited to 20%.
Voltage drop during motor steady state conditions shall be limited to 5%.
7.5.2 Steady state voltage drop
(a) For AC Circuits
The voltage drop under normal conditions is given as
Vd% = Ib * Lc *
Rc * Cos + Xc * Sin
* K * 100
1000 * Vn * n
Where,
Vd%
= Percentage voltage drop at normal conditions (V)
Ib
= Rated current at full load at normal condition (A)
Lc
= Cable length (mtrs)
Rc
= Resistance of cable conductor (Ohm/km)
Xc
= Reactance of cable conductor (Ohm/km)
= Coefficient determined by the kind of system. i.e. 3 in case of three-phase system and
2 in case of single-phase system
Cos = Power factor of the load at normal conditions
Vn
= Nominal voltage (V)
= no of cables running in parallel per phase
(b) For DC Circuits
Vd% = 2 * I * Lc * Rdc * 100
Vdc * n
Where,
Vd%
= Percentage voltage drop at normal conditions (V)
= Line current at normal conditions (A)
Vdc
= System voltage (V)
Lc
= Cable length (mtrs)
Rdc
= DC conductor resistance at maximum rated conductor temperature (Ohm/km)
= no of runs
7.5.3 Voltage Drop under motor starting conditions
The voltage drop for three-phase motors under starting condition is verified with the formula:
Vd,st% = Ibst * Lc *
Rc * Cos st + Xc * Sin st
* K * 100
1000 * Vn * n
Where,
Vd,st%
= Percentage voltage drop at motor starting time (V)
Ib,st
= Motor current at starting time (A)
Lc
= Cable length (mtrs)
= Resistance of cable conductor (Ohm/km)
Xc
= Reactance of cable conductor (Ohm/km)
= Coefficient determined by the kind of system i.e.
3 in case of three-phase system and
2 in case of single-phase system
Cos st = Power factor at motor starting time
Vn
= Nominal voltage (V)
= no of cables running in parallel per phase
8.
REFERENCE DOCUMENTS
9.
TYPICAL CALCULATIONS
CABLE SIZING CALCULATIONS FOR MOTOR 48-P-4805A/B RATED 0.18 KW (LAID IN AIR)
Parameters Considered
Motor rating for 48-P-4805A/B
: 0.18 KW
Full load power factor
: 0.76
Full load efficiency
: 0.64
Continuous Current carrying Capacity
Full Load current (FLA) Ib
Load (KW) x1000
3 System Voltage x Power Factor x efficiency
Full Load current (FLA) Ib
0.18 x1000
3 x0.415x1000 x 0.76 x 0.64
Full Load current (FLA) Ib
0.5149 Amps.
0.52 Amps.
Cable Site Rating = (Cable Current Carrying Capacity) X (Group Derating Factor)
X (No. of Conductor per phase)
Group derating factor for cables laid in ground is calculated as shown below.
a
b
Rating factor for Ambient Air temperature of 55 C
Kt
=0.94
Rating factor for grouping of cables (6 cables with
Kg
=0.57
0.15m spacing)
Overall admissible ampacity correction factor Df= Kt x Ks x Kd x Kg=0.54
The current carrying capacity for 3C X 2.5 mm2 Cu cable is 27 Amps. (Refer Appendix-1)
So,
Cable Site Rating = (27) x (0.54) x (1)
Cable Site Rating = 14.04 Amps
Say
= 14.5 Amps
De-Rated current is greater than full load current (Ib) of the motor.
Voltage Drop under normal condition
The maximum voltage drop allowed from Distribution board to motor is
Under Starting condition
= 20%
Under Running Condition
= 2%
For Motor
Cos (running)
0.76
Cos (starting)
0.108
Running Current (FLA) Ib
0.6 A
Starting Current (Ist)
4.2 A (Locked Rotor Current (LRC) shall be limited to 7 times
Full Load current (FLC.)
For 3C x 2.5mm2 Armoured XLPE (Cu) cable
Resistance
9.45 (ohm/Km)
Reactance
0.097 (ohm/Km)
Length (L)
15 mtrs
Vd%
3* Ib x (Rc *Cos+ Xc *Sin)* Lc * 100
1000 * Vn * n
Vd%
3 * 0.515 x (9.45 *0.76+ 0.097 *0.65)* 15 * 100
415 * 1 *1000
Vd%
0.023 %
Voltage Drop under motor starting conditions
Vd, st% =
3x Istx (Rc Cosst+ Xc Sinst) x Lc x 100%
1000 * Vn * n
Vd, st% =
3x 3.6 x (9.45x0.108+ 0.097 x0.99) x 15 x 100%
415 * 1000 * 1
Vd, st % =
0.025 %
Voltage for motor under steady-state and starting conditions is lower than permissible value.
Short Circuit withstand criteria
The short circuit rating for 1 second is given by
ISC = 143 * S
T
ISC = 143* 2.5
1
ISC = 357.5 A
Say ISC = 358 A
Since,
-
Derated current is greater than full load current
Voltage drop from distribution board to Motor is lower than the permissible values
Hence, 1 x 3C x 2.5 mm2 Cu XLPE cable meets the requirement.
9.1 SUMMARY
Cable size & Voltage drop calculations for Barrel Unloading Pumps, Agitator Pumps and
Power Outlets are summarized in attached Appendix-4.
10.
NOTES
1. Ambient temperature is considered as per the Doc no: Environmental Design Data.
2. Maximum conductor operating temperature of 90C is considered as per IEC60364-5-52
Table.52.4 which gives the maximum operating temperatures for types of insulation.
3. Motor KW ratings considered suitably with reference to Chemical Injection Package (Corrosion
Inhibitor) pump data sheet.
4. Power factor, efficiency and locked rotor current (starting current) for Motors are considered
suitably.
5. Cable length from LV Switchgear to Motors is considered as 15mtrs.
6. Cable lengths considered suitably, shall be revised as per approved Cable tray layout.
7. MCCB short-circuit current and time of respective loads is considered by referring approved
vendors.
